601
Macromolecules 1987,20, 601-608
Electronic Spectra of Polysilanest L. A. Harrah* and J. M. Zeigler* Sandia National Laboratories, Albuquerque, New Mexico 87185. Received July 24, 1986
ABSTRACT: The absorption, polarized absorption, and fluorescence spectra of a variety of high molecular weight polysilanes in solution are reported. The first absorption maximum of alkyl-substitutedpolysilanes shifts linearly to lower energies with increasing substituent size. We suggest that this shift stems from steric interference of the substituents which results in straining of the Si-Si backbone bonds and/or a change of the backbone conformational preference. Strong parallel dichroism in stretch-oriented (di-n-hexylsilane), is observed. Aryl substitution gives rise to additional shifts in the first absorption band to lower energies and introduces parallel dichroism in oriented materials when the substituents are phenyl or p-anisyl but not with P-naphthyl. No evidence for vibrational fine structure exists in either absorption or fluorescence in the high polymer spectra even at low temperature. These results are interpreted RS evidence for the first transition being u u* in the alkyl-substituted polysilanes, mixed x-u-u*-likein phenyl- and p-anisyl-substituted x*-like for @naphthyl substitutions. polysilanes, and K
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Introduction Recently high molecular weight polysilanes have received considerable attention as precursors to silicon carbide fiber production,l as photoinitiators in alkene polymerization,2 as photoresists: and as photo conductor^.^ Although all known polysilanes are photosensitive in solution, a variety of solid-state photochemical behavior, depending upon the substituent nature, has been ob~ e r v e d . We ~ ? ~have studied the photophysical properties of high molecular weight polysilanes with a variety of substituents to both identify the precursors to photochemistry and provide insight into the mechanisms of charge and energy transport in this class of interesting new polymers. Studies of the UV spectra of oligosilanes were first reported over 20 years ago. Gilman, Atwell, and Schwebkel discussed the ultraviolet spectra of a series of silanes with from two to six dimethylsilane units having phenyl terminations and observed that these behaved similarly to the a,w-diphenylpolyenes in that their absorption position shifted to the red as the silicon backbone length in~reased.~ Thef also examined the disilanes with increasing phenyl substitutions from zero to six phenyls replacing methyl substituents. In these studies, they observed an apparent conjugation of the aromatic ring with the silicon-silicon linkage and suggested that the activity of this linkage arose from the interaction of the vacant silicon d orbitals with the aromatic (or olefinic) a orbitals of the substituent. Hague and Princelo examined the spectra of a variety of phenyl-substituted silanes and interpreted the spectral data in terms of a a a(dSi) transition from the ring a levels to a a level constructed from the vacant d a orbitals of the silicon chain or states formed from mixtures of these a levels. Pitt and co-workers’l showed that HMO theory was successful in treating the energy dependence of the absorption maxima of permethyl polysilanes having from 3 to 10 silane units. If the assumption was made that the upper states of the transition contained some aC-C character when phenyl or vinyl groups were substituted, the same treatment was effective in predicting their absorption maxima. This result suggested that, at least for the short-chain polysilanes, the transition origin was associated with the silicon-silicon bonding orbitals. Ionization potentials for methyl-substituted silanes having up
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‘This work performed at Sandia National Laboratories was supported by the U S . Department of Energy under Contract Number DEAC04-76DP00789.
0024-9297/87/2220-0601$01,50/0
to eight silicons were obtained13 and also could be fit with simple molecular orbital methods, assuming that these ionization potentials gave the ground-state energy for a delocalized bonding orbital.14 The results indicated that the shift in HOMO energy accounted for about 60% of the magnitude of the spectral red shifts of the permethylated silanes with increasing chain length. UV photoelectron spectra,15ionization potentials, and half-wave reduction potentials16 were used to develop a model of bonding in aryl-substituted oligosilanes. In this work it is concluded that the HOMO’S are mixtures of the delocalized silicon chain o-bonding orbitals with the abonding orbitals of the aryl substituents. For phenyl substitution, this model suggests that the HOMO is principally aSi-Si and the first transition a-a*(ring), but, with the lower ionization potential of p-anisyl or naphthyl substituents, the HOMO becomes more *-like and the corresponding spectra arise from a(ring) **(ring) transitions. More recently, higher molecular weight oligomers of permethylated12J7J8silanes have been synthesized. In studies of the permethyl silanes with chain lengths extending to 24 silane atoms, Boberski and Allred12 found that the behavior of not only the lowest energy transition but the next observable band as well could be fit with the same parameters using the Sandorfy “C” method. They proposed that both bands originate from transitions that involve delocalized antibonding or bonding orbitals. In the higher members of this series the two transitions become nearly energetically degenerate and would be expected to be unresolvable in the corresponding high polymer. The X-ray photoelectron spectra and NMR spectra of both linear and cyclic oligosilanes18~2G29 indicate that the a-bonding orbitals are delocalized in these small molecules. The behavior of the alkyl polysilanes and cyclopolysilanes suggests that the HOMO can be interpreted as a delocalized a-bonding orbital in the larger linear and cyclic oligosilanes in much the same way as with the lower molecular weight analogues. Absorption spectra of high molecular weight homo- and copolysilanes with methyl, alkyl, and aryl side c h a i n ~ ~ ~ ~ J ” ~ ~ have been published recently, as has the dependence of the first absorption maximum on chain length for two of these high polymers. Absorption and emission spectra of poly(dimethylsi1ane-co-methyl-( 1-naphthy1)silane) in solution have been examined by Todesco and Kamat.6 Naphthalene-like monomer and excimer emission was observed for both the singlet and triplet states. These workers also presented evidence that photoscission of the
0 1987 American Chemical Society
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602 Harrah and Zeigler
Macromolecules, Vol. 20, No. 3, 1987
polysilane backbone occurs mainly a t naphthyl-bearing silicon a t o m s to give silyl radicals. Solid-state absorption and emission spectra of a fully alkyl-substituted polysilane, poly(isopropylmethylsi1aneco-n-propylmethylsilane) have been discussed by Zeigler e t al.3a In this case, a narrow high quantum yield (& = 0.65) fluorescence was seen. However, t h e phosphorescent emission was weak (df broad, and strongly vibrationally coupled t o Si-C vibrations. On the basis of these observations, s t a t e s in the triplet manifold were suggested as the likely precursors to photochemical bond scission in t h i s "self-developing" resist material. A theoretical t r e a t m e n t of t h e p a r e n t polysilane electronic s t r u c t u r e has been given by Takeda and Matsumoto30 a n d suggests that the HOMO in this material is purely Si-Si u-bonding, but t h e LUMO is a mixture of both Si-Si and Si-H antibonding character. This analysis also indicates t h e presence of a filled band very close t o t h e HOMO for infinite chains with an allowed transition t o t h e LUMO. The existence of this second bonding level m a y account for the two transitions which merge with increasing chain length in t h e permethylated silane telomers. W e have synthesized a variety of high polysilanes with both alkyl and aryl substituents a n d examined n o t only their absorption spectra in solution but also their spectra a s n e a t solid films a n d in oriented polymer hosts. T h e i r fluorescence spectra were also obtained to gain additional insight i n t o t h e nature of the first excited state of these intriguing systems. Kepler e t al.4 have shown that b o t h alkyl- a n d aryl-substituted polysilanes photoconduct with a quantum efficiency of ~ 1 at%room temperature. The photocarriers a r e holes and exhibit high mobility (.=IO-* cm2/ (V-s)) for amorphous materials. No electron mobility could be detected. This behavior is not in accord with t h e expectations of t h e simple delocalized bonding model for t h e small molecule^^^^^^ or the nature of t h e first transition in those telomers, in which electrons and holes would b e expected t o have similar mobilities. W e propose that t h e evidence favors a first transition in the alkyl-substituted polysilanes that is primarily 0 o* in nature overlapped b y a transition originating in a second bonding level close t o t h e HOMO. For the aryl-substituted materials, t h e transitions are similar in origin to t h e alkyl compounds but both the HOMO and LUMO contain some R character derived from mixing with t h e ring states. For low energy differences between the ring R a n d R* levels, t h e first R*(ring). transition becomes principally R
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Experimental Section General. Hydrocarbon solvents were dried over CaS04 and then refluxed and distilled under argon from NaK alloy. Ether solvents were predried over CaSO,, distilled under argon from Na/benzophenone ketyl to remove residual water and peroxides, and stored under argon until use. A representative synthetic procedure for preparation and polymerization3* of a noncommercial monomer follows below. Monomer synthesis and polymerizations were performed in dried, N2-purged glassware. All NMR, MS) monomers and polymers were fully characterized (Et, and gave satisfactory microanalytical results (within 0.3% of theoretical for C, H, and Si). Absorption and emission spectra were obtained on polymers purified by three to four precipitations from toluene or tetrahydrofuran with a nonsolvent (which was specific to the polymer being prepared). For the selected narrow molecular weight poly(phenylmethylsilane), a purified, broad molecular weight range preparation was fractionated on a gel permeation chromatograph in tetrahydrofuran, and the collected fractions were examined without isolating the solid polymers. Solution spectra at room temperature were taken in freshly deoxygenated, distilled tetrahydrofuran or 2-methyltetrahvdrofuran for the low-temperature studies.
The spectra were observed by using a combination absorption spectrometer-fluorimeter of our own design. The instrument consists of a xenon (or mercury-xenon) arc lamp, 0.2-m f4.0 Czerny-Turner monochromator/illuminator, and a sample compartment where the sample can be viewed a t right angles to the exciting beam or can be illuminated at 180" with a deuterium arc lamp. Detection uses a 0.22-m double Czerny-Turner monochromator equipped with a cooled gallium arsenide cathode photomultiplier. Both photon counting and direct current detection electronics are simultaneously available. In addition, the transmitted exciting beam is continuously monitored by a separate in-line detector to compensate for fluctuations in the lamp output. Data gathering is controlled by an LSI-11 microcomputer which scans the monochromator and reads the data via a gated 32-bit counter or analogue-to-digitalconverter. The results are corrected for both excitation intensity and analyzer throughput and recorded on floppy disks for further study. The sensitivity of the monochromator-phototube combination as a function of wavelength was calibrated by using a ribbon filament tungsten source standardized by NBS. This sensitivity curve is used to correct all fluorescence band shapes. Low-temperature spectra were obtained by using gaseous nitrogen cooling of the sample chamber. Absorption spectra were obtained by scanning the straight-line deuterium source in the absence of right-angle excitation and can be obtained without movement of the sample under study. All solution samples studied were extremely photosensitive to light in the region of absorbance. Neutral density filters (OD 2.0-4.0) were used in the exciting beam to minimize photodecomposition during spectrum scans. The absorbance near the long-wavelength transition was very sensitive to the degree of photolysis, and the excitation light transmission was continuously monitored to assure that the spectra were not distorted by photodecomposition. Sufficient reduction of exciting light was used so that the transmitted light varied less than about 1%and the fluorescence spectra on repetitive scans reproduced within instrumental error. Sample solutions were protected from room light prior to spectral determinations. The neat solid films of poly(phenylmethylsi1ane) were.cast on quartz plates from tetrahydrofuran solution in thicknesses ranging from 6400 A to greater than 30 gm. The polymer-host-supported films were cast from solutions containing the polymer host (poly(methy1 methacrylate) (PMMA) for most systems, poly(nbutyl methacrylate) for poly(phenylmethylsi1ane)). The dried host film containing =0.1% polysilane was oriented by straining to =700% near its TBand examined a t room temperature. Fluorescence quantum yields were measured by comparison with the fluorescence intensity of p-terphenyl in the same solvent. The quantum yield for p-terphenyl was taken as 0.93.32 (4-Methoxyphenyl)methyldichlorosilane. A solution of 24.57 g (105 mmol) 4-iodoanisole (Aldrich) in 100 mL of dry benzene was prepared in a dried, argon-purged 3-neck flask equipped with an Allihn condenser, a pressure-equalizing dropping funnel, a gas inlet, and a stirring bar. To this was added dropwise a t ambient temperature 100 mL of 2.1 M tert-butyllithium (210 mmol) in pentane. A milky white precipitate formed, and a slight exotherm was evident. The mixture was heated a t reflux for 2 h after completion of the addition and then allowed to cool to room temperature. Meanwhile, a second flask equipped with a condenser, pressure-equalizing funnel, and gas inlet was readied, and a solution of 15.68 g (105 mmol) of freshly distilled methyltrichlorosilane in 100 mL of dry benzene was prepared in it. The 4-methoxyphenyllithium solution was transferred via double-tipped needles to the dropping funnel and added dropwise to the silane solution. After 2 h, the reaction mixture gave a negative Gilman test for lithium reagent and was worked up by filtration in a nitrogen atmosphere followed by removal of solvent at reduced pressure. The resulting oil was fractionally distilled at a pressure of 1mmHg to afford 7.6 g (33%)of colorless product, bp 89-92 "C (lit. bp 96 "C (3 mmHg)).39 Poly(4-methoxyphenylmethylsilane). A 40 wt % sodium dispersion (2.69 g, 46.9 mmol) was added dropwise to a refluxing solution of 4.71 g (21.3 mmol) of (4-methoxypheny1)methyldichlorosilane in 100 mL of dry toluene contained in a foil-covered reaction vessel. The addition was maintained at the highest rate allowed by the strongly exothermic reaction. Reflux was continued
Electronic Spectra of Polysilanes 603
Macromolecules, Vol. 20, No. 3, 1987
Table I Summary of Absorption and Emission Spectral Data for Poly(dialkylsi1anes) no. of
substituents n-hexyl, methyl cyclohexyl, methyl n-dodecyl,methyl di-n-hexyl 6-phenethyl,methyl
dimethyl"
X-1
3060 3170 3100 3180 3050 2960
6500 6540 6340 10500 9950 45500
F,
emax2
Amax2
2240 2200 2140 2280
3360 3450 3370 3420 3375
2930 2870 17140 28000
bn
units
0.61 0.42 0.67
660 40 155 5100 6700 24
OFrom extrapolated values, ref 12.
for 4 h after completion of the Na addition, whereupon the dark blue slurry was cooled to room temperature and sufficient methanol added cautiously to react with the excess sodium. A volume of saturated aqueous NaHCO, equal to that of the reaction mixture was added in a single portion, and, after complete dissolution of the blue NaCl byproduct had taken place, the layers were separated. Removal of solvent from the organic phase afforded 5.08 g of crude product as a viscous oil. Two precipitations of this from toluene with hexane and one from tetrahydrofuran (THF) with methanol gave 130 mg of pure white powdered polymer. Size exclusion chromatography of this material (THF, 1.0 mL/min, Zorbax bimodal column set) indicated a modal molecular weight relative to polystyrene standards of 30 OOO with a shoulder at 5000: lH NMR (CDC13) 6 -0.2 (br s,3 H, Si-CH3), 3.8 (br s,3 H, OCHJ, 6.6 (br m, 4 H, aryl H); IR (KBr) 3010,2940, and 685 cm-'. Anal. 2850,1595,1275,1250,1090,1030,810,750, C, H, Si.
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Results Spectra of Alkyl Polysilanes. The observed solution spectra of the alkyl polysilanes are summarized in Table
I. The spectra of poly(@-phenethylmethylsilane) are included with the data on alkyl-substituted silanes since both the absorption and fluorescence spectra and their maxima and widths suggest that the aromatic ring insulated from the chain by the two methylene groups is not important in determining the states involved in the transition. Figure 1compares the spectra of poly(@-phenethylmethylsilane) with poly(n-hexylmethylsilane). The positions of the first absorption maxima in alkyl polysilanes suggest that, in the high molecular weight limit, the transition energy is determined primarily by the size of the substituent groups. Figure 2 shows the relationship between absorption position and an estimated size based on the sum of the radii of the groups attached to the silicon-bonded carbons. For this estimate, only the first three carbons and their associated hydrogens are considered, and the conformation is taken to separate the third carbon the maximum distance from its associated silicon. This estimate ignores the remainder of the chain and may underestimate somewhat the effective size of the dodecyl group. Nevertheless, it does suggest that the limiting long-chain transition energy is related linearly to an effective side-group size. Note that, again, the size of the @-phenethyland methyl substituents places the corresponding polymer clearly in this series. The widths of both the absorption and emission spectra in poly(dialkylsi1anes) are substantially greater than those of high molecular weight poly(phenylmethylsi1ane) and poly@-anisylmethylsilane). No evidence for vibrational fine structure is shown in any of the high polymer solution spectra. Below -100 "C both the absorption and emission bands narrow in the alkyl-substituted polymers. The spectral width of these transitions is probably due to both the existence of two closely spaced lower levels, perhaps the two transitions of the permethyl series unresolved, and inhomogeneous broadening related to conformational energy dependence of the transitions. In dilute solutions a t approximately -31 "C, poly(di-n-hexylsilane)
WAVELENGTH (nm)
Figure 1. Absorption and emission spectra of (a) poly(@-phenethylmethylsilane)in THF and (b) poly(n-hexylmethylsilane)in THF. 10
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604 Harrah and Zeigler
Macromolecules, Vol. 20, No. 3, 1987
Table I1 Summary of Absorption and Emission Spectral Data for Poly(arylmethylsi1anes)
substituents phenyl, methyl p-anisyl, methyl &naphthyl, methyl
0.40 W
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Figure 3. Polarized absorption spectra of a stretch-oriented (draw ratio -20) film of poly(di-n-hexylsilane)on poly(methy1meth-
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acrylate). asymmetrically substituted alkyl polysilanes and alkyl polysilane copolymers also exhibit either substantial thermochromism or rod-coil transitions a t low temperatures, depending upon solvent, with band narrowing and bathochromic shifts as the temperature is lowered. A few polysilanes (e.g., poly(cyclohexylmethylsi1ane)) exhibit hypsochromic shifts with decreased line widths on cooling which are observable both in absorption and emission. We have suggested3" that this effect may be due to a change in the conformationalpreference of the polymer chain from trans in those polysilanes showing bathochromic shifts to gauche or pseudogauche in those polysilanes displaying hypsochromic shift behavior. Unsymmetrically substituted polysilanes show greater line widths than the symmetrically substituted polymers. Since the rod form must be substantially more ordered than the random coil and has considerably narrower spectra, the wide bands in the alkyl-substituted polysilanes are likely to result primarily from inhomogeneous (conformational) broadening. Polarized absorption spectra were taken for both poly(p-phenethylmethylsilane) and poly(cyclohexylmethy1silane) by doping the polymers into high molecular weight poly(methy1 methacrylate). No visible scattering was observed in these doped films either before or after straining at Tg to -700%. The resulting films exhibited no dichroism from 275 to 400 nm. However, when an approximately 2000-A-thick film of poly(di-n-hexylsilane) cast onto a 100-pm film of poly(methy1 methacrylate) was similarly oriented (draw ratio >20), it displayed the expected strong dichroism parallel to the draw direction, presumably parallel to the chain. The spectrum from this film (Figure 3) was that of a nearly 100% crystalline material and, subsequent to the draw, showed no detectable absorption in perpendicularly polarized light (i.e., essentially complete parallel orientation was achieved). This observation suggests that the failure to detect orientation in the host solutions of the other poly(dialkylsi1anes) examined is either a result of segregation of the solute or, more likely, that the solute is present in a globular, noninterpenetrated conformation which cannot undergo sufficient orientation for the dichroism to be detectable by these techniques.
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Figure 4. Absorption and emission spectra of aryl polysilanes in THF solutions: (a) poly(phenylmethylei1ane); (b) poly(panisylmethylsilane);(c) poly@-naphthylmethylsilane).
Spectra of Aryl Polysilanes. The solution spectra of the three aryl polysilanes studied are summarized in Table I1 and their spectra shown in Figure 4. Here, two distinctly different spectral types are apparent. For both phenyl and p-anisyl substitution, the spectra are narrow both in absorption and emission, while @-naphthylsubstitution gives weak absorption and a very broad emission spectrum. This broad emission spectrum is similar in shape to the spectrum observed from poly(2-vinyl-
Macromolecules, Vol. 20, No. 3, 1987
Electronic Spectra of Polysilanes 605 1.o
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Figure 5. Polarized absorption spectra of aryl polysilanes in oriented polymer hosts: (solid curve) parallel orientation; (dotted curve) perpendicular orientation; (dashed curve) parallel-perpendicular);(a) poly(phenylmethylsi1ane) in poly(n-butyl methacrylate); (b) poly@-anisylmethylsilane) in PMMA; (c) poly(Pnaphthylmethylsilane) in PMMA. naphthalene) in 2-methyltetrahydrofuran. This emission has been attributed to intramolecular excimer formationM prior to emission. Near the high-energy region a small subsidiary shoulder is apparent, again similar to the case of poly(2-vinylnaphthalene)and attributed in that polymer to fluorescence from unassociated excited naphthalene moieties. The polarized absorption spectra of these three polymers were examined in a high molecular weight polymer host following strain alignment. Both poly@-anisylmethylsilane) and poly(@-naphthylmethylsilane) were miscible at the 0.1% level with poly(methy1 methacrylate). The phenyl, methyl-substituted polymer precipitated in this host, but was found compatible with poly(n-butyl methacrylate) and was observed in that host. The spectra ob-
Figure 6. Absorption spectrum,emission spectrum, and emission quantum yield of cast neat poly(phenylmethylsi1ane). The emission quantum yield scale is linear with a value of 0.5 at the emission peak. tained are shown in Figure 5 together with the difference ell - e l . Poly(phenylmethylsi1ane) and poly@-anisylmethylsilane) exhibit substantial dichroism with the maximum transition moment in the strain direction (presumably the chain axis direction). Poly@-naphthylmethylsilane) shows no detectable dichroism. Clearly, either the upper or lower orbital type has changed in going from p-anisyl to naphthyl substitution. Both polymers displaying dichroism exhibit a maximum in the difference spectrum which does not coincide with the maximum in the first absorption band. This observation, coupled with the substantial width difference between absorption and emission, suggests that the first absorption band is complex, consisting of at least two components with different origins. The relative quantum yield for fluorescence in poly(phenylmethylsilane)thick f i i s ( ~ 5 Fm) 0 was determined as a function of wavelength in the region of the first absorption. Figure 6 shows these spectra and quantum yields. The absorption is taken from a thin film (6500 A) and the fluorescence quantum yield from a thick sample to simplify yield measurement in the weakly absorbing tail. The quantum yield rises by a factor of =3 from the short-wavelengthedge of the band to the long-wavelength tail, again suggesting that the band is resolvable into a t least two transitions. An alternative rationalization for this observation, that longer wavelength excitation selects polymer chains with higher degrees of delocalization, thereby leading to more efficient fluorescence, accounts for the shift of the polarization difference curve with respect to the absorption maximum only if the longer alltrans chain segments are selectively oriented in the drawing process. We have examined the quantum yields in solution for poly(phenylmethylsi1ane) a t modal molecular weights of 450 000 and near 30 000. The high molecular weight material excited at 302 nm emits with a yield of 0.112 f 0.01 and the lower molecular weight with a yield of 0.093 f 0.01. The nearly equal quantum yields observed support our suggestion of the complex band origin for the wavelength dependence of the quantum yield and seem to rule out the photoselection alternative. An attempt was made to resolve the two postulated transitions by subtracting progressively larger fractions of a band with the shape of e,, - t i from the parallel spectrum for the p-anisylmethyl polymer in poly(methy1 methacrylate). A reasonable fit was obtained when the intensity of the two components was approximately equal (Figure 7 ) . A similar procedure performed with the pheny1,methyl polymer gives about a
606 Harrah and Zeigler 0.6
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Macromolecules, Vol. 20, No. 3, 1987 ,
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Discussion The spectral behavior of the alkyl polysilanes suggests that the lowest energy absorption arises from a u u* transition from a delocalized silicon-ilicon bonding orbital to a delocalized antibonding level. The widths of the fluorescence spectra relative to the absorption spectra suggest that the observed absorption band may be complex; perhaps it is a combination of the two transitions observed to be coalescing in the discrete permethyl telomers.12 In the alkanes, the lowest excited state has been assigned to a Rydberg level which lies very close to the first antibonding u - ~ t a t e The . ~ ~ presence of an aromatic ring, even removed from the chain by an insulating ethylene linkage, should still substantially lower a Rydberg excitation energy but the consistency of the spectrum of poly(P-phenethylmethylsilane) with the similarly sized poly(n-hexylmethylsilane) would seem to exclude the a-Rydberg origin of the first transition in polysilanes. Recent calculations on the electronic states of the parent polysilane (SiH2),30indicate that the first transition is to a u* level consisting of a mixture of silicon-silicon antibonding and silicon-substituent antibonding character. These arguments and the observed spectra would seem to favor an assignment of the first band to a u u* transition. If the observed band is a combination of the two transitions observed in the permethyl telomers, both are likely to be u u*. Our finding of strong parallel dichroism in stretch-oriented poly(di-n-hexylsilane) is completely consistent with and a necessary consequence of the u u* interpretation for the transition. The high quantum yield for fluorescence (up to 67 % for poly(n-dodecylmethylsilane)) is curious in light of the extreme photosensitivity of these polymers. The fluorescence yield and lack of vibrational coupling in the fluorescence emission at low temperatures suggest that the singlet state is highly delocalized. These observations argue for a photochemically stable material, yet poly(diorganosilanes) are usually high photosensitive in solution and solid state. The low quantum yield of phosphorescence in low-temperature glasses, short (=1ms) triplet lifetime, and strong vibrational coupling in the triplet imply a high degree of excitation localization in the triplet manifold relative to that observed for the singlet states, suggesting that triplet states are probably responsible for photochemical backbone scission in these materials. Photochemical depolymerization quantum yields were measured in solid films of poly(cyclohexylmethy1silaneco-dimethylsilane) to be =6, consistent with a previously proposed chain decomposition mechanism involving multiple silylene expulsions from the radical chain ends generated by the initial backbone scission.3a The large shift associated with the rod-coil transition in poly(di-n-hexylsilane)solutions as well as the absorption narrowing in the rod form indicates that a large fraction of the bandwidth in the alkyl polymers may result from conformational inhomogeneity, a not unexpected result even for u-bonded systems.36 The substituent size dependence apparently is due to steric interference between pendant groups either 1,2 or 1,333"to each other, which can result in Si-Si bond lengthening and/or a change in the equilibrium content of the trans and gauche conformers. The spectra of the poly(arylsi1anes)can be interpreted in the following manner: both phenyl- and p-anisyl-sub-
-
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zm
naphthyl substituent. In time-resolved measurements, the pheny1,methyl polymer exhibits weak phosphorescence, consistent with the extremely weak phosphorescence reported for poly(isopropylmethylsi1ane-co-n-propylmethylsilane) .3a
270
310
290
330
350
370
390
WAVELENGTH (nm)
Figure 7. Spectrum of poly@-anisylmethylsilane)in PMMA taken with polarization parallel to draw direction showing the result of partial resolution of the first absorption band by subtraction of the difference spectrum,parallel-perpendicular: (solid curve) parallel orientation spectrum-difference spectrum; (dotted curve) difference spectrum). I
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DEGREE OF POLYMERIZATION
Figure 8. Transition energy in THF solutions of poly(pheny1methylsilane) vs. degree of polymerization. Solid line is calculated from E = a + 2 p cos (J?r/N+ 1) (see ref 17).
3:l ratio for the two components. Narrow molecular weight fractions of trimethylsilylterminated poly(phenylmethylsi1ane) were separated and their solution spectra measured. As the molecular weight increases, the transition energy decreases as in the permethyl series,12but the decrease is not well fit by the MO p r e d i ~ t i o n s ' ~in J ~ the region beyond =12 chain units (Figure 8). Among the aryl polysilanes, we have characterized the thermochromic behavior only for poly(phenylmethy1silane). Solutions of this material exhibit a slight hypsochromic shift and a decrease in line width of absorption on cooling. Whether this behavior is associated with conformational changes in the polymer backbone, as in the alkyl polysilanes, or stems from some other source is not known a t this time. Fluorescence spectra of poly(phenylmethylsi1ane)were obtained over the temperature range 299-77 K in 2methyltetrahydrofuran solution. The full width a t halfmaximum decreases from 17.0 nm a t 299 K to 7 nm at 77 K with no observable fine structure. The poly@-anisylmethylsilane) and poly(/3-naphthylmethylsilane)polymers were also examined in rigid 2-methyltetrahydrofuran at 77 K. In steady-state measurements, neither the pheny1,methyl nor the p-anisyl polymer showed resolvable phosphorescence, while the &naphthyl polymer phosphoresced with an emission characteristic of the isolated
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Electronic Spectra of Polysilanes 607
Macromolecules, Vol. 20, No. 3, 1987 stituted polymers have similar ground and excited states; in the naphthyl-substituted material, either the ground state or the excited state or both have changed character. The absorption spectrum of the @-naphthylpolymer is substantially different in character from either phenyl- or p-anisyl-substituted polymers. Several overlapping transitions are found in the /3-naphthyl material, while only three well-separated bands are evident in the phenyl and p-anisyl spectra. The fluorescence of the @-naphthylsubstituted polymer occurs from both naphthalene-like excimer and naphthalene-like monomer. Phosphorescence is strong and is naphthalene-like. The transition in the @-naphthylpolymer, therefore, must be predominantly a **(ring). This conclusion is in accord with the analyses of Pitt et al.15J6and the recent observations by Todesco and Kamat6 on poly(dimethylsi1ane-co-1-naphthylmethylsilane). The behavior of the p-anisyl polymer, by Pitt's analysis, would be predicted to follow that of the @-naphthyl-substituted material rather than that of the phenyl-substituted material. Yet the spectral shifts, shapes, and polarizations clearly associate the p-anisyl behavior with the phenyl. From the arguments of Pitt and Bock15 and Pitt et a1.,16 the poly(phenylmethylsi1ane) transition should be u **(ring) or *(ring) u*, depending on the relative mixtures of character in the excited state. The parameter affecting ground-state mixing of the a(ring) and u Si-Si bonding levels is related to the substituent ionization potential. Both p-anisyl and @-naphthylgroups have ionization potentials of about 8.12 eV,16 and the ground state would be expected to be roughly the same mixture of ring and chain levels in the high polymer. Certainly the phenyl polymer with a side-group ionization potential of 9.25 eV15 would have substantially different ground-state character than either the p-anisyl or the @-naphthyl. However, the nature of the transition depends on the relative mixing in the upper level as well as in the ground state. Here it is evident that raising the side-group ground-state energy alone did not result in a change in the transition character but lowering the excited-state energy did. If the ground state is a(ring)-like in character in the @-naphthyl and p-anisyl polymers, the excited-state character changes from u* Si-Si-like in phenyl and p-anisyl polymers to a*(ring)-like in @-naphthyl.A transition from a localized ring state to a delocalized chain state would be expected to have the major transition moment parallel to the polymer chain (as would the a a* predicted by Pitt et al.).15J6 While the polarization observation alone cannot distinguish between the a(ring)-u*(chain) and the u(chain) a*(ring) possibilities, when coupled with the fluorescence and absorption spectra character changes, it provides convincing support for the argument that the transition is a u*. We have carried out simple MO calculations on a short-chain model of the poly(phenylmethylsi1ane) that also indicate a flow of electron density from the ring to the chain on excitation. It is possible that the observed dichroism results not from chain alignment in our experiments but from side-group organization. Two observations argue against this possibility: (1)the largest side group, @-naphthyl,shows no dichroism and (2) the direction of the maximum transition moment should be perpendicular to the chain orientation for a a*-like trans i t i o n ~ ~for ' the polysilanes with phenyl, p-anisyl, and naphthyl side chains. It should be noted that our assignment of the nature of the transitions in polysilanes is supported by recently reported photoelectron spectroscopy studies of Loubriel and Zeigler41on polysilanes which show that alkyl and naphthyl side group binding energies are
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nearly unperturbed by the presence of the silicon backbone. The phenyl binding energy is strongly affected, indicative of considerable cross-hybridization of the phenyl with the polysilane backbone. We conclude that the first transition in alkyl-substituted polysilanes arises from a delocalized cr u* transition which is inhomogeneouslybroadened by chain conformational defects in solution and amorphous solids. The first transition in the small-ring aryl polysilanes is probably ?r u* in character (charge transfer, ring to chain) and is less affected by conformationaldefects. In the amorphous solids this transition may lead to a charge-transfer exciton band. In the large fused aromatic ring substituents (8naphthyl), the transition is best characterized as a localized T-T* naphthalene-like excitation.
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Acknowledgment. We thank R. G. Kepler, Z. G. Soos, and K. S. Schweizer for many helpful discussions that served to clarify our observations and motivate further study of these extremely interesting polymers. Registry No. (4-Methoxyphenyl)methyldichlorosilane, 18236-55-0; 4-iodoanisole, 696-62-8; methyltrichlorosilane, 75-79-6; poly(4-methoxyphenylmethyldichlorosilane) (homopolymer), 97464-14-7;poly(hexylmethyldichlorosi1ane) (homopolymer), 88002-83-9; poly(cyclohexylmethy1dichlorosilane)(homopalper), 88002-85-1; poly(dodecylmethyldichlorosi1ane)(homopolymer), 88002-84-0; poly(dihexyldichlorosi1ane)(homopolymer), 9703667-4;poly( (@-phenethy1)methyldichlorosilane) (homopolymer), 88002-80-6;poly(dimethyldichlorosi1ane) (homopolymer), 30107-43-8;poly(phenylmethyldichlorosi1ane) (homopolymer), 31324-77-3;poly((/3-naphthyl)methyldichlorosilane) (homopolymer), 100894-75-5.
References and Notes Yajima, S. Am. Ceram. SOC.Bull. 1983,62,893and references therein. Peterson, D. C.; Wolff, A. R.; West, R. Presented at the 17th Organosilicon Symposium, 1983, Fargo, ND (unpublished). Wolff, A. R. Ph.D. Dissertation, 1984. (a) Zeigler, J. M.; Harrah, L. A,; Johnson, A. W. SPZE Adu. Resist Technol. Proc. ZI 1985,539,166-174. (b) Zeigler, J. M.; Harrah, L. A. U.S. Patents 4 587 205 and 4 588 801. Kepler, R. G.;Zeigler, J. M.; Harrah, L. A,; Kurtz, S. R. Bull. Am. Phys. SOC.1983,28,362. Kepler, R. G.; Zeigler, J. M.; Hanah, L. A.; Kurtz, S. R., submitted for publication to Phys. Rev. B: Condens. Matter. West, R.; David, L. D.; Djurovich, P. I.; Stearley, K. L.; Srinivasan, K. S. V.; Hu, H. J. Am. Chem. SOC.1981, 103, 7352-7354. Todesco, R. V.; Kamat, P. V. Macromolecules 1986, 19, 196-200. Gilman, H.; Atwell, W. H.; Schwebke, G. L. Chem. Ind. (London) 1964, 1063. Gilman, H.; Atwell, W. H.; Schwebke, G. L. J. Organometal. Chem. 1964,371-372. Hausser, K. W.; Kuhn, R.; Smakula, A. Z. Phys. Chem., Abt. E 1935,29,384. Hague, D. N.; Prince, R. M. Proc. Chem. SOC.,London 1965, 4690. Pitt, C. G.;Jones, L. L.; Ramsey, B. G. J. Am. Chem. SOC. 1967,89,5471. Boberski, W. G.;Allred, A. L. J. Organometal. Chem. 1975,88, 65-72. Pitt, C. G.; Bursey, M. M.; Rogerson, P. F. J . Am. Chem. SOC. 1970,92,519-522. Koopmans, T. Physics 1934,1, 104. Pitt, C. G.; Bock, H. J. Chem. SOC.,Chem. Commun. 1972,29. Pitt, C. G.; Carey, R. N.; Toren, E. C., Jr. J . Am. Chem. SOC. 1972,94,3806-3811. Brough, L. F.; West, R. J. Am. Chem. SOC.1981,103, 3049. West, R.; Carberry, E. Science (Washington,D.C.)1975,189, 179-186. Trujillo, R. E. J. Organometal Chem. 1980, 198,C27-C28. Trefonas, P.; Djurovich, P. I.; Zhang, X. H.; West, R.; Miller, R. D.; Hofer, D. J. Polym. Sci., Polym. Lett. Ed. 1983,21, 819-822. West, R.; David, L. D.; Djurovich, P. I.; Yu, H.; Sinclair, R. Ceram. Bull. 1983,62,899. Zhang, X.H.; West, R. J . Polym. Sci. 1984,22,225-238.
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(23) Zhang, X. H.; West, R. J. Polym. Sci. 1984, 22, 159-170. (24) Trefonas, P.; West, R.; Miller, R. D.; Hofer, D. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 828-829. (25) Miller, R. D.; Hofer, D.; Wilson, C. G.; Trefonas, P.; West, R. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1984, 25(1), 307-308. (26) Stanislawski, D. A.; West, R. J. Organometal. Chem. 1981,204, 307-314. (27) Stanislawski, D. A.; West, R. J. Organometal. Chem. 1981,204, 295-305. (28) West, R.; Kramer, F. A.; Carberry, E. J . Organometal. Chem. 1967,8, 79-85. (29) Brough, L. F.; West, R. J . Am. Chem. SOC.1981, 103, 3049-3056. (30) . , Takeda. K.: Matsumoto. N. Phvs. Reu. B: Condens. Matter 1984,30, 5871-5876. ' (31) Zeigler, J. M. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1986, 27(1), 109-110. (32) Berlman. I. B. Handbook of Fluorescence SDectra o.f Organic Compounds, 2nd ed.; Academic: New York, 1971. (33) (a) Harrah, L. A.; Zeigler, J. M. J. Polym. Sci., Polym. Lett. Ed. 1985,23,209-211. (b) Schweizer, K. S. Chem. Phys. Lett.
(34) (35) (36) (37) (38)
(39) (40)
(41)
1986,125, 118-122. (c) Zeigler, J. M.; Adolf, D.; Harrah, L. A. Presented at the 20th Organosilicon Symposium, Tarrytown, NY, 1986; abstract P-2.25. Frank, C. W.; Harrah, L. A. J. Chem. Phys. 1974, 61, 1526-1541. Robin, M. B. Higher Lying Excited States of Polyatomic Molecules; Academic: New York, 1974; Vol. 1, pp 104-105. Dewar, M. J. S. J. Am. Chem. SOC.1984, 106, 669-682. Klevens, H. B.; Platt, J. R. J . Chem. Phys. 1949,17,470-481. Caution: All solvents must be thoroughly dry and monomers pure to polymerize dihalosilanes safely. The use of wet solvents and/or impure monomers leads to dangerous induction periods in the polymerization reaction that can result in a violent and uncontrollable exotherm when polymerization finally initiates. Aleksandrova, Y. A,; Kotleva, N. S.; Sorokina, R. A,; Bakhurina, L. M.; Nikitina, T. S.; Panov, Y. M.; Pravednikov, A. N. Vysokomol. Soyedin., Ser. A 1969,11, 2470. The curve shown in Figure 8 was normalized to pass through the highest molecular weight point in the asymptote. Loubriel, G.; Zeigler, J. M. Phys. Rev. B: Condens. Matter 1986, 33, 4203-4206.
Solid-state NMR Study of the Hexamethylenetetramine Curing of Phenolic Resins Galen
R. Hatfield and Gary E. Maciel*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523. Received August 19, 1986
ABSTRACT The curing of a phenolic resin by hexamethylenetetramine (HMTA) is elucidated through the use of and 15N CP/MAS NMR spectroscopy. The cure was shown to result in increased cross-linking, with the bridging carbons originating from the curing agent. Intermediates involved in the curing process are identified, with species having structures of the benzoxazine and tribenzylamine types being the major components. The results are compared to previous studies on model systems, and the power of solid-state NMR to probe chemistry in complex macromolecules is again clearly demonstrated.
Introduction In 1907 a technique for processing phenol-formaldehyde resins, the first wholly synthetic commercial polymer material,' was developed by Baekeland. Today, phenolic resins have great commercial applications, especially as molding compounds, coatings, and industrial bonding resins. Details of the curing process are responsible for many of the important physical and mechanical characteristics of these materials. Thus, an explicit knowledge of the curing process is essential for understanding and improving the use of phenolic resins. Although the curing of phenol-formaldehyde resins has received a great deal of attention,l-12 most of the previous studies have suffered from experimentallimitations, chiefly due to solubility limitations, dissolution uncertainties, and heavy reliance on model systems. Recent studies13-16have demonstrated the use of solid-state 13C NMR with cross polarization (CP) and magic-angle spinning (MAS)17-19for systems warranting such concerns. The 13C CP/MAS technique has proven to be a powerful tool for structural elucidation in macromolecular systems. It is the application of this technique to the investigation of the hexamethylenetetramine (HMTA) curing of a phenol-formaldehyde resin of the novolak type on which this paper focuses. Experimental Section NMR Measurements. 13C CP/MAS spectra were obtained a t 25.1 MHz on a home-built spectrometer that utilizes a Nicolet 1180 data system and a Nalorac 4.7-T magnet operated a t half field. Spectra were obtained a t numerous contact times, ranging
* Author to whom correspondence should be addressed. 0024-9297 / 8712220-0608$01.50 / 0
from 0.5 to 4 ms, and the relative peak intensities showed no major dependence on contact time over that range. Spectra of the cured resins presented here were obtained with a 2-ms contact time and a 1-9 repetition time. Samples were spun at 3.2-3.5 kHz, using spinners of the Windmill type.m The magic angle was adjusted to within 0.lo by using the 7eBr spectrum of KBr placed in a spinnereZ1 15NCP/MAS spectra were obtained a t 20.3 MHz on a modified wide-bore Nicolet NT-200 spectrometer. The NMR parameters used to accumulate the spectra presented here were a 4-ms contact time and a 15-s repetition time. Samples were spun at 2.2 kHz, using bullet-type spinners,22and the magic angle was again set by the KBr method. Proton-decoupled solution-state 13Cspectra were obtained on an IBM WP-2OOSY spectrometer at a carbon frequency of 50.3 MHz, while 15Nsolution-state spectra were obtained at 36.5 MHz on a Nicolet "-360 spectrometer. All 13Cspectra were referenced externally to tetramethylsilane at 0 ppm and all I5N spectra to liquid NH3, also a t 0 ppm. Samples. The test resin used in this study was a novolak phenolic resin prepared at the Durez Division of the Occidental Chemical Corp. under conditions believed to produce a "random resin". The details of the 13C CP/MAS spectrum of this resin have been reported previou~ly.'~The test resin was cured by heating a mixture of 90% resin plus 10% HMTA for 1h a t the temperature noted in the text. Both the 13C- and 15N-labeled HMTA used in this study were prepared by the method of Bulusu and c o - ~ o r k e r s using , ~ ~ 99% 13C-enriched CH20 or 99% 15Nenriched ammonium acetate, respectively. Model compounds used in this study were either obtained commercially from Aldrich Chemical Co. or synthesized by using literature preparations, as noted in the text.
Results and Discussion The key types of molecular structures that a r e known or suspected to be involved or of interest in this study are 0 1987 American Chemical Society
